1. Field of the Invention
This disclosure relates to a branching unit and arrangement for delivering a respiratory gas of a subject.
2. Description of Related Art
Total Lung capacity (TLC) is dependent upon many factors such as a weight, a sex, an age and an activity. For example, females tend to have a 20-25% lower capacity than males. Tall people tend to have a larger total lung capacity than shorter people. Heavy smokers have a drastically lower TLC than nonsmokers. Some people, such as elite athletes, have a TLC well above average.
Tidal volume (TV) is an amount of an air inspired or taken into the lungs in a single breath. TV is also dependent on the sex, size, height, age and a health etc. of a patient. In general TV also decreases as the size of the patient decreases. In an average healthy adult, TV is about 400-600 ml whereas in an average healthy neonate, that measures 3.5-4 kg and is 50 cm tall, TV is approximately 25-50 ml. On the other hand, in an average premature neonate that measures only 500 grams TV is only about 2-3.5 ml. TV of a smaller patient's is very difficult to measure, but it can be approximated to 4-7 ml/kg, applying a general rule of thumb for approximating the TV of the human lung. In practice the TV of the patient suffering pulmonary system deficiency is normally much less than the approximation gives.
When the patient is mechanically ventilated with a conventional ventilator, an endotracheal tube is placed into a trachea so that it goes through oral or nasal cavity and larynx. The other end of the endotracheal tube is connected to a breathing circuit Y-piece through a luer type connector. If the patient is gas monitored with a mainstream or sidestream gas analyzer, an airway adapter used for sampling the breathing gas that is analyzed by the gas analyzer is normally connected between connectors of the endotracheal tube and the breathing circuit Y-piece. During an inspiration the fresh breathing gas including higher oxygen (O2) concentration flows into the patients lungs through an inspiratory limb of the breathing circuit Y-piece, the airway adapter, the endotracheal tube and their connectors, then to a trachea, a bronchus, a bronchi, bronchioles and finally reaching an alveoli deep in the lungs, where all the gas exchange actually occurs. Carbon dioxide (CO2) molecules in a hemoglobin of a blood flowing in tiny blood vessels around the alveoli are replaced with O2 molecules in the fresh breathing gas through the thin walls of the alveoli. O2 molecules take their place in the hemoglobin, whereas CO2 molecules flow out from the patient within the used expired breathing gas, through the same path as the fresh gas came in during the inspiration. Thus a gas concentration of the breathing gas measured by the gas analyzer is somewhat proportional to the gas concentration in the blood.
A volume in a space between a connection of the inspiratory and expiratory limbs of the Y-piece and the patient's mouth or nose, a beginning of oral and nasal cavities, is called a mechanical dead volume or dead space, whereas the volume in a space between patient s mouth or nose and the entrance of alveoli is called an anatomical dead volume. The part of the lung that is injured or damaged for some reason and does not participate for the gas exchange is called more specific a physical dead volume. It is obvious that as the used breathing gas flows out from the patient's lungs through the expiratory limb during expiration, a part of the used gas newer exits a pulmonary system, as well as the patient side of the breathing circuit, but remains in the mechanical and anatomical dead volume. Then as the fresh gas is inspired in to the lungs through the inspiratory limb the used gas already in the anatomical and mechanical dead volume flows into the lungs before the fresh gas. The used gas fills up some or all of the alveoli depending on a ratio of the dead volume and TV or at least mixes up with the fresh gas decreasing the concentration of O2 as well as increasing the concentration of CO2 in the lungs, which in turn decreases the gas exchange in the alveoli. This means that the larger the dead space, the larger the volume of the used gas, with a low O2 and high CO2 concentration, that flows back to the patients lungs during the inspiration and worse the gas exchange in the alveoli. In other words, if the total dead volume were larger than TV or as large as TV, the patient would not get any fresh gas into the lungs, but respires the used gas back and forth in the dead volume. In practice a diffusion of gases assists the gas exchange over the dead volume little, especially when there is some movement of gases such as a high frequency ventilation evolved, but the overall gas exchange in the alveoli would be lethal or dangerously poor anyway.
The anatomical dead volume is almost impossible to reduce, but it is proportional to the size and the physical condition of the patient. The mechanical dead volume depends on a breathing circuit design, an inner diameter of a tubing, connectors and additional accessories, such as sidestream and mainstream gas analyzers Obviously the mechanical dead volume is more critical for smaller patients with smaller TV or patients suffering barotraumas etc., which also decrease TV. In practice the sidestream gas analyzing is not suitable for the patients with very small TV, since in addition to a dead volume increment caused by the airway adapter, conventional sidestream gas analyzers “steal” sample gas from the inspratory and expiratory gas flow, thus decreasing the gas exchange in the alveoli. Furthermore respiration rates (RR) of smaller patients are higher, up to 150 breaths/minute or even more, which is well above the measurement range of the conventional sidestream gas measurement technology, compared to adult patients with RR less than 60 breaths/minute.
Although the conventional mainstream gas analyzers are able to measure higher RR more than 60 breaths/minute with high TV, the analyzer dead volume is even more than that of sidestream gas analyzers. Together with the Y-piece dead space, where the mainstream analyzer is connected to, is much too high to be used with smaller patients. Thus at the moment there does not exist a proper breathing gas concentration analyzing technique for smaller patients. The high overall dead volume together and non existing breathing gas analyzing are also reasons why a conventional ventilation cannot be used in many cases or at least it is difficult or even dangerous to use. Due to the weaknesses of conventional ventilation patients are more likely ventilated with high frequency ventilators (HFV) with RR up to 3000. These ventilators do not have the conventional inspiration and expiration phase as normal respiration, but the gas exchange in the alveoli is ensured through the diffusion of gases. HFV has it own drawbacks in addition that the gas diffusion type high frequency ventilation also makes it impossible to measure breathing gas concentrations comparable to the gas concentration in the alveoli with any conventional gas analyzer technology.
FIG. 1 shows an exploded schematic view of the patient side part of the conventional breathing circuit consisting of the endotracheal tube 1, the Y-piece 2 and a combination of the conventional mainstream type airway adapter 32 and the gas analyzer 3 known in prior art.—The Y-piece comprises three limbs. The inner diameter of the limb that connects to endotracheal tube is approximately 15 mm, whereby a cross-sectional inner area is approximately 180 mm2. The inner diameter of those limbs that connect to ventilator is approximately 19 mm, whereby a cross-sectional inner area is approximately 280 mm2. The airway adapter 32 comprises a sampling chamber 33 in the middle of a female luer connector 34 and a male luer connector 35. The connectors 34 and 35 are conventional standard size connectors, which connection diameter is 15 mm or a cross-sectional area of approximately 180 mm2. The inner diameter of the male luer connector is 13-13.5 mm, a cross-sectional area of approximately 135-145 mm2 and the length 17-28 mm. Female luer connectors fit on male luer connectors in every connection of the breathing circuit, thus the inner diameter is conical approximately from 14.5 to 15.5 mm. The airway adapter 32 is placed into a cavity 36 in the conventional analyzer body 31 so that breathing gases flowing through the breathing circuit and through the sampling chamber 33 in airway adapter 32 can be analyzed by the analyzer body 31. The gas analyzer 3 is connected between the endotracheal tube 1 and the Y-piece 2 through its airway adapter 32. The airway adapter 32 connects through the male connector 35 to a female connector 21 of the Y-piece 2 and similarly the female connector 34 of the airway adapter 32 connects to a male connection of separate connector 11, which further connects to the endotracheal tube 1 through a tubular connection.
The inner diameter of endotracheal tube 1 can vary from 2 mm to 10 mm or more or in terms of a cross-sectional area approximately from 3 to 79 mm2 or more and the length can vary from 150 mm to 250 mm or more depending on the patient it is connected to. In general the inner diameter (ID) of the endotracheal tube 1 increases as the age (or the size proportional to the age) of the patient increases. In general the smaller the patient the smaller the endotracheal tube 1 used. Table 1 below shows some recommendations for the use of endotracheal tubes with different aged patients from manufacturers.
TABLE 1Uncuffed tubeCuffed tubeAgeID [mm]ID [mm][years]2.02.53.03.0<1 (<3 kg)3.53.51-2 {close oversize brace} Neonatal4.04.02-44.54.54-65.05.06-8 {close oversize brace} Pediatric5.55.5 8-106.06.010-126.56.512-147.07.014-167.5>16 {close oversize brace} Adult8.08.59.09.510.0
Thus the connectors 11 are conventionally used to connect very different size of the endotracheal tubes 1 to one size of the airway adapter 32, which means that each size of endotracheal tube 1 needs a separate connector 11 connected to it. Other end of the connector 11 is a standard size male connector that fits in to the female connector 34 of the airway adapter 32 and the other end is tubular connector that fits to it's respective endotracheal tube. The total length of the connector 11 is approximately 31 mm, the length of the tubular part approximately 9 mm and the length of the male connector approximately 22 mm.
FIG. 2 shows a schematic view of the breathing circuit already shown in FIG. 1 as all the separate parts are connected together. The mechanical dead volume of the conventional breathing circuit is the volume between places 13 and 24, shown with dashed lines in FIG. 2. The place 13 on the endotracheal tube 1 is the place where the endotracheal tube 1 comes out from the nasal or oral cavity of the patient and the place 24 is a cross section where the inspiratory limb 22 and the expiratory limb 23 of the Y-piece 2 connect to the connector 21. The volume of the endotracheal tube 1 consists of the volume of the connector 11 and the volume of the endotracheal tube 1 sticking out from the patient. The dead volume of the connector 11 is approximately 2-3 ml alone. The volume of the airway adapter 32 depends on the length and the inner diameter of the sampling chamber 33 added with the volume what is left of the connector 34 as the connector 11 connects to it and with a volume of connector 35, which all depend on the design of different manufacturers. The inner diameter and the length of the sampling chamber 33 becomes from the technical requirements of the gas measurement. That then determines the length and the outer diameter of airway adapter 32, which in turn determines the size of an analyzer body 31 of the gas analyzer 3, which fits on the airway adapter 32. The cross sectional shape of the channel of conventional airway adapters, in to the direction of the gas flow, is rectangular. Regardless of the size of the patient the airway adapter is connected, the width of the channel is 8-10 mm in to the direction in which the gas is analyzed, whereas in to the other direction it is 10-13 mm. The cross-sectional area of the channel is thus approximately 80-130 mm2. The length of the channel, in to the direction of the gas flow, varies between 22-32 mm. The dead volume of the conventional airway adapters is usually much more than 1 ml, around 4-5 ml. The dead volume of the conventional Y-piece 2 becomes mainly from the volume of the connector 21, which is approximately 2-3 ml. Theoretically, a small dead volume around the cross section of the inspiratory limb 22 and the expiratory limb 23 where the inspiratory and expiratory gases mix can be added to the total dead volume of the Y-piece 2.
Conventional mainstream gas analyzers, as well as airway adapters connected to them, are big and heavy, which is one of the disadvantages when used with the smaller patient. Another disadvantage are badly designed connections between different parts of the breathing circuit, such as a step like changes in the flow path that cause turbulences in to the breathing gas as well as gas pockets between connectors that cause further mixing, but also rapid decrease in the flow velocity of the gas which increases the response time. As an example the cross-sectional area of the endotracheal tube, which inner diameter is 2 mm, is approximately 3 mm2, whereas the cross-sectional area of the male connector at the end of the endotracheal tube is approximately 110 mm2. Furthermore the cross-sectional area of the female connector of the airway adapter, where the endotracheal tube is connected to is approximately 180 mm2. As can be seen the ratio between cross sectional areas within the breathing gas flow path is enormous. Every step like change causes turbulences, which mix up inspiratory and expiratory edges and gas pockets cause additional gas concentration offset as the old gas with different gas concentration accumulated into the gas pockets mixes up with the new gas. Furthermore as the inspiratory gas comes out from the patient through the tiny endotracheal tubing in to the large volume of the male connector and female connector of the airway adapter the flow velocity of the gas decelerates rapidly just before the sampling chamber, where the gas concentration measurement occurs. In addition to the turbulence the step like changes cause the deceleration of the gas flow degrades the response time to gas concentration changes even more, which can be seen especially as the RR increases. The biggest step like the change is the difference in the inner diameter between the endotracheal tube used with smaller patients (2-4.5 mm, cross-sectional area of 3-16 mm2) and the female connector of airway adapter (15 mm with cross-sectional area of 180 mm2). Rest of the connections generates smaller steps into the breathing gas flow path, but the airway adapter also includes gas pockets. Although the problem is biggest with smaller diameter endotracheal tubes, the same problem occurs also with larger endotracheal tubes. For example the cross sectional area of the endotracheal tube, which inner diameter is 7 mm, is approximately 38 mm2. This is still about ¼ compared to the cross sectional area of 180 mm2 of the female connector at the airway adapter, where the endotracheal tube is connected to.
However, one of the biggest disadvantages for the patient is the large dead volume of Y-pieces and conventional airway adapters and the whole patient side part of the conventional breathing circuit where the gas analyzer is connected. The total dead volume of such conventional breathing circuit, the Y-piece, the airway adapter and endotracheal tube with connector, as was described earlier and shown in FIGS. 1 and 2, which is further connected to the patient that weights 1 kg through the endotracheal tube with the inner diameter of 2.5 mm is at best more than 7 ml. This is approximately one to two times higher than TV of the patient in this example, which is approximately 4-7 ml. This means that the patient in the above example rebreaths the used gas and is more likely to suffer from the poor gas exchange than to get better treatment
Thus at the moment there does exist no suitable patient side part of the breathing circuit as well as Y-piece and airway adapter for the breathing gas concentration measurement, at least for small neonatal and premature neonatal patients who have small TV, but the existing configuration could be better for larger patients as well.
The large dead volume of the conventional breathing circuits, the Y-pieces and the airway adapters is one of the biggest and common disadvantages in respiratory care, since the large dead volume interferes the gas exchange in the lungs as the breathing circuit rather than the lung is ventilated and the patient is rebreathing the used gas. Thus small, intubated patients are often ventilated with high frequency ventilators (HFV) with very high respiration rates up to 3000 breaths/min. The high frequency ventilation does not comprise the normal inspiration and expiration phase, but is more like a vibration and the diffusion of the gases, which makes it impossible to gas monitor the patient with the conventional gas analyzing techniques. Moreover the high frequency ventilators are noise and their functionality disputed. The only way to analyze the gas exchange in the lungs is to measure CO2 and O2 concentration from the blood through blood samples or a transcutaneous measurement. The blood sampling is very stressful and even dangerous for the small patient whose blood volume is very small. The trancutaneous measurement has its own weaknesses, such as a need for connections to the patient's skin so that oxygen in the blood just under the skin can be measured. Especially the skin of premature neonates is very thin and fragile and thus the measurement is not very often used. Connections also come loose from the patient easily and the technique is such that it heats up the patient at the connections, which place thus has to be changed. The blood sampling is not a real time measurement, as it has to be analyzed in the laboratory, which in turn causes a long time delay into an acute patient care. As there is no real time measurement to analyze the gas exchange of small patients, they are usually ventilated insufficiently, which causes different trauma for the patient and a longer time to recover.